The present invention relates to an anisotropic light-scattering film which is useful for assuring a uniform light emission on a display device (a flat-type surface display device), a method of producing the same, and a display device utilizing the film. More particularly, the invention relates to an anisotropic light-scattering film which is useful for transmission type (mode) or reflection type (mode) liquid crystal display or projection television.
Among surface display devices (illumination devices) such as a liquid crystal display device, there is known a reflection type display device comprising a light reflecting layer formed on the back of a liquid crystal display module so that the light incident on the front surface is reflected by the light reflecting layer. Also known is a backlight type (transmission type) display device comprising a fluorescent tube disposed behind or on a lateral side of a liquid crystal display module. In the arrangement where the fluorescent tube is disposed on a lateral side, a light guide for causing the light from the fluorescent tube to emerge in the frontal direction is disposed on the back side of the liquid crystal display module [Japanese Patent Application Laid-Open No. 333141/1998 (JP-A-10-333141)].
However, in such liquid crystal display devices, the uniformity of the displayed image is sometimes low. For example, when the display device described in the specification of Japanese Patent Application Laid-Open No. 333141/1998 referred to above is used, the luminance distribution is not uniform in the direction normal to the longitudinal axis of the fluorescent tube, with a consequent marked variation in luminance, as is obvious from FIG. 14, FIG. 15 and FIG. 16 appended to the above specification.
For this reason, a light-scattering film (diffuser or diffusing film) is frequently used to diffuse the light from the fluorescent tube or the reflected light from the light reflecting layer to assure a uniform luminance. As the light-scattering film, a light-scattering film comprising a polycarbonate or polyester base film which is transparent and highly heat-resistant, and containing refractive microfine particles (resin beads) or light-transmitting inorganic microfine particles incorporated therein by coating or otherwise is generally employed.
Recent years have seen an increasing demand for such light-scattering films as the light-scattering film for the backlight component of the backlight type liquid crystal display device. The light-scattering film for backlight use is usually interposed between the backlight (cold cathode tube) and the liquid crystal layer to homogenize the light emitted from the cold cathode tube. However, when the scatting of light is too large, no sufficient emission luminance can be obtained. Therefore, an optical element such as a prismatic lens is interposed between the light-scattering film (diffuser) and the liquid crystal layer to thereby refract the diffused light so that the light will be incident perpendicularly on the liquid crystal display surface, thus upholding the luminance.
As the surface display device [i.e., a display device the image display area of which is a flat surface (a flat type display device)] which is equipped with a diffuser, the device illustrated in FIG. 4 is known. This device comprises a surface display module 45 (particularly a transmission type liquid crystal display module) and at least one fluorescent discharge tube (cold cathode tube) 41 which is adapted to illuminate the module from its back side. Disposed on the back side of the fluorescent discharged tube 41 is a reflector 42 for reflecting the light advancing toward the back side. Moreover a diffuser 43 for diffusing light to uniformly illuminate the module 45 is interposed between the fluorescent discharged tube 41 and the module 45 and a prism sheet 44 is disposed on the module side of the diffuser 43. This surface display module 45, in the case of a liquid crystal display module, comprises a first polarizing film 46a, a first glass substrate 47a, a first electrode 48a on the glass substrate, a first alignment layer 49a on the electrode, a liquid crystal layer 50, a second alignment layer 49b, a second electrode 48b, a color filter 51, a second glass substrate 47b, and a second polarizing film 46b as successively built up (laminated) in the order mentioned. In such a display device, the display module can be directly illuminated from the back side by the built-in fluorescent tube (cold cathode tube) 41. However, even when a diffuser (light-scattering film) is used, an uneven emission (luminance) distribution in the direction normal to the longitudinal axis of the fluorescent tube is inevitable, causing a streak pattern to appear, although the emission distribution in the longitudinal direction is uniform.
Furthermore, the device including a light guide can be constructed by using the backlight unit illustrated in FIG. 5 as the backlight system of the surface display device of FIG. 4. This backlight unit has a fluorescent tube (cold cathode tube) 51 and a reflector member 55 disposed in parallel with the fluorescent tube, with a light guide 54 having a diffuser 53 at top and a reflector 52 at bottom being disposed in the direction of light emission from the fluorescent tube. The lower part of the light guide 54 is inclined so that the light from the fluorescent tube can be reflected in an upward direction. The light emerging in the direction of the top of the light guide 54 is diffused by the diffuser 53 and incident on the surface display module (not shown) constructed (laminated) on the diffuser.
When such a backlight unit is used, in contrast to the backlight unit or component of FIG. 4, the emission distribution may appear uniform over the surface but a detailed observation of the emission distribution reveals that the distribution is still not as uniform as desired. Thus, as shown in FIGS. 6 and 7, the emission distribution (luminance distribution) in the longitudinal (axial) direction (x-direction) of the fluorescent tube (cold cathode tube) 51 is small as it is the case in the device of FIG. 4 but the emission from the fluorescent tube (cold cathode tube) in the y-direction which is normal to the x-direction is repeatedly reflected by the reflector 52 and advances in the z-direction (the direction in which the liquid crystal display module is disposed) which is perpendicular to the xy plane so that the emission distribution (luminance distribution) in the y-direction is distorted (in a zigzag pattern), thus failing to assure sufficient uniformity.
Thus, in the usual backlight type display device, the emission distribution (luminance distribution) in the direction normal to the longitudinal direction (X-direction) of the fluorescent tube is not uniform and a streak-like directionality (linear dark areas) is produced in the emission distribution. Moreover, even when a light-scattering film containing microfine particles is used, it is inevitable from its isotropy of light scattering that the luminance in a certain direction (the direction of disposition of the fluorescent tube, the streaking direction, X-direction) is unduly lowered.
Japanese Patent Application Laid-Open No. 142843/1999 (JP-A-11-142843) describes a technology such that a dot pattern for scattering light is formed in rows perpendicular to the light source on the surface of the light guide. However, even with this contrivance, linear dark areas (a streak pattern) are observed in the direction of disposition of the fluorescent tube.
Japanese Patent Application Laid-Open No. 261171/1995 (JP-A-7-261171) describes a reflection type liquid crystal display device comprising a pair of glass substrates, an electrode formed on each of the opposed surfaces of the glass substrates, a liquid crystal sealed interposed between the electrodes, and a polarizing film formed on (laminated) the outer surface of the external one of the pair of glass electrodes, with a light-scattering layer composed of two or more kinds of resins differing from each other in the index of refraction and forming mutually segregated (separated) phases being disposed on the surface of the polarizing film. In this literature, it is mentioned that the polarizing film is coated or printed with a mixture of two or more kinds of resins in a solvent to form the light-scattering layer.
Furthermore, as a reflection type liquid crystal display device (or a reflection type liquid crystal module), the device (or module) illustrated in FIG. 8 is also known. This reflection type display module comprises a pair of glass substrates 81a, 81b, a pair of electrodes 82a, 82b formed on the opposed surfaces of the glass substrates, and a liquid crystal 87 interposed between the electrodes forming the pair, the electrode 82a formed on the glass substrate 81a on the back side constituting light-reflective pixel electrodes and a color filer 84 being interposed between the glass substrate 81b on the front side and the electrode 82b. In addition, a phase difference layer 86 is constructed (laminated) through a polarizing layer 85 on the front surface of the glass substrate 81b on the front side. In this reflection type liquid crystal display module, a diffuser 83 is formed on the front surface (the front surface of the phase difference layer 86) to constitute a reflection type liquid crystal display device. Since, in the reflection type liquid crystal display device, one polarizing layer 85 is situated on the front side of the liquid crystal cell, unlike in the transmission type display device having a built-in lamp (the backlight type liquid crystal display device), the incident light which is incident on the front surface of the device (external light) is diffused by the diffuser 83 and enters into the liquid crystal cell and then is reflected by the reflective electrode (reflector) 82a within the liquid crystal cell and diffused by the diffuser 83. Therefore, the data or image displayed on the display module can be visually recognized from any angle without loss of luminance without the provision of a lamp (light) but utilizing external light.
In the reflection type liquid crystal display device, however, if the light diffusing power or ability of the diffuser is too great, the incident light and the reflected light are randomly reflected in a large measure by the diffuser so that the clarity of displayed data is sometimes sacrificed.
Meanwhile, Japanese Patent Application Laid-Open No. 314522/1992 (JP-A-4-314522) describes an anisotropic light-scattering material comprising a transparent matrix and a (particulate) transparent substance which is morphologically anisotropic and differing in the index of refraction from the transparent matrix as uniformly dispersed in the matrix in a positional relation shifted in an orderly and mutually parallel manner. In this literature, it is disclosed that the anisotropic light-scattering material such that the morphologically anisotropic transparent substance has a particle size of 0.5 to 70 xcexcm and an aspect ratio of not less than 10, preferably 15 to 30, is of value as a lenticular lens for the projection television screen. There, a light-scattering film featuring the aspect ratio of about 10 to 25 with a minor axis dimension of about 1 to 2 xcexcm is manufactured by a method which comprises kneading a low-melting low-density polyethylene for the transparent matrix resin with a polystyrene or a styrene-acrylonitrile copolymer for the (particulate) transparent substance, extruding the resulting resin composition, and cooling the molten resin extruded in the form of a sheet under a large draft applied in the direction of extrusion.
However, even when this anisotropic light-scattering material is applied to the backlight type display device, the uniformity of emission distribution is still inadequate. Moreover, this anisotropic light-scattering material is inadequate in heat resistance as well.
The present invention has for its object to provide an anisotropic light-scattering film assuring a uniform surface emission with close tolerances and without compromise in luminance, a method of producing the same, and a display device (particularly a liquid crystal display device) utilizing the film.
It is another object of the present invention to provide an anisotropic light-scattering film giving a uniform surface emission easily even if the light from a light source has an anisotropy of emission distribution (distribution of luminance), a method of producing the same and a display device (particularly a liquid crystal display device) utilizing the film.
It is still another object of the present invention to provide an anisotropic light-scattering film having a good anisotropy of light scattering despite its high transparency, a method of producing the same, and a display device (particularly a transmission type liquid crystal display device) utilizing the film.
It is another yet object of the present invention to provide a reflection type liquid crystal display device which upholds the clarity of displayed data and has a strong display directionality.
It is a further object of the present invention to provide a lenticular lens having a good anisotropy of light scattering as well as high heat resistance.
The inventors of the present invention did much research to accomplish the above objects and found that, in a liquid crystal display device equipped with a light projectors having an anisotropy (directionality) of emission distribution and a light diffusing film, the ratio of the light scattering characteristic Fx(xcex8) in one direction of the film and the light scattering characteristic Fy(xcex8) in the direction normal thereto over the range of scattering angle xcex8=4 to 30xc2x0 is a major factor in the uniformization of the luminance distribution of emerging light and that when the relation of Fy(xcex8)/Fx(xcex8) greater than 5 holds over the range of scattering angle xcex8=4 to 30xc2x0 , the luminance distribution can be uniformized without compromise in the luminance of the image displayed. The present invention has been developed on the basis of the above findings.
Thus, the anisotropic light-scattering film of the present invention is capable of scattering an incident light in the direction of advance of the light and, in the scattering characteristic F(xcex8) relevant to the relation between the scattering angle xcex8 and the intensity of scattered light F, satisfies the following relation over the range of xcex8=4 to 30xc2x0.
Fy(xcex8)/Fx(xcex8) greater than 5
where Fx(xcex8) represents the scattering characteristic in the direction of the X-axis of the film and Fy(xcex8) represents the scattering characteristic in the direction of the Y-axis of the film.
This film can be comprised of a continuous phase and a particulate discontinuous phase (dispersed phase or dispersoid) which differ from each other in the index of refraction by not less than 0.001 and is characterized in that the mean aspect ratio of dispersed phase (dispersoid) particles is greater than 1 (for example about 5 to 500) and that the major axes of dispersed phase particles are oriented usually in the direction of the X-axis of the film. The mean dimension of the minor axes of dispersed phase particles may for example be about 0.1 to 10 xcexcm. Since, with such a film, light can be diffused with high anisotropy, the emission distribution can be uniformized without compromise in luminance even when a light projector means having an anisotropy of emission distribution (e.g. a tubular light projector) is used in conjunction.
As the resins for constituting the continuous phase and dispersed phase, transparent resins can be employed. For example, the continuous phase may be formed by using a crystalline olefin resin (e.g. a polypropylene-series resin), while the dispersed phase may be composed of a noncrystalline polyester-series resin. This anisotropic light-scattering film may further contain a compatibilizing agent (e.g. an epoxidized diene-series block copolymer). The relative amount of the continuous phase and dispersed phase may for example be [former/latter]=about 99/1 to 50/50 (weight ratio) and the relative amount of the dispersed phase and compatibilizing agent may for example be [former/latter]=about 99/1 to 50/50 (weight ratio). The film may be formed with surface irregularities extending in the direction of its X-axis.
The above film can be produced by melt-blending and extruding the continuous-phase resin and dispersed phase resin and carrying out an orientation treatment for orienting the dispersed phase. The orientation treatment includes a method of producing a film under constant application of a draft and a method in which the solidified film is monoaxially stretched (e.g. using a calender roll). This orientation treatment (for example, the stretching) may be carried out at a temperature beyond the melting point or glass transition point of the dispersed phase-forming resin.
The present invention further encompasses a display device comprising a display module, a tubular light projector means for projecting light toward the module, and the anisotropic light-scattering film as disposed forwardly of the projector means. The light-scattering film is disposed with its X-axis aligned with the longitudinal direction of the projector means. Furthermore, the present invention may be embodied as a reflection type liquid crystal display device including the light-scattering film or as a lenticular lens for projection television which is constituted by the light-scattering film.
Throughout this specification, the term xe2x80x9cfilmxe2x80x9d is used without regard to thickness, thus meaning a sheet as well.